In recent years, especially lightening of the type of structures as mentioned above with respect to their weight has been especially desired and emphasized, and thinning of the structures has accordingly been promoted. Soft steel, stainless steel, aluminum alloys or the like are usually applied to the plates of the panels. With the promotion of the thinning of the structures, "sag" of the plate after welding and assemblage inevitably arises, and problems are posed as to the local strength of a "sagging" part of the panel as well as the external appearance of the panel.
With the objective of eliminating the aforenoted welding distortion, there have accordingly been disclosed techniques wherein a large number of sheets of a standard shape are formed into an elongated plate by means of welding the sheets together, whereupon under the state in which the plate is loaded with a tensile stress, it is attached to a skeleton (refer to, for example, the official gazette of Japanese Patent Application Publication No. 53-39261), and additional techniques wherein the above plate formed to be elongate is loaded with the tensile stress and is also preheated so as to thermally expand the same, whereupon the plate is attached to the skeleton (refer to, for example, the official gazzette of Japanese Patent Application Publication No. 54-20185). Both of these techniques, however, require a large-scale tensioning device and heating device and involve enormous installation costs as well as a large number of process steps. As another drawback, it is not easy to precisely control the predetermined heating temperature. There has also been provided a technique wherein a plate of comparatively small size is disposed within a heating box, and the whole plate is heated to a predetermined temperature and is thereafter welded to a skeleton (refer to, for example, the official gazette of Japanese Patent Application Laid-open No. 60-64791). However, the heating box is complicated in structure. Moreover, as the plate becomes larger in size, the heating box needs to be proportionately larger in size, whereby problems of a large floor space and a workshop of large area are incurred and the installation cost becomes high.
Furthermore, there has also been disclosed a technique wherein a plate having an opening for a window or a door is placed upon a heat insulating material laid upon a surface table, and a heating means such as a heater or a hot iron plate is disposed upon the plate, while the heating means is compressed by a pressure application means such as a press, so as to heat the plate in contact therewith, the plate being thereafter welded to a skeleton (refer to, for example, the official gazzette of Japanese Patent Application Publication No. 53-39262). This technique, however, has the drawback that the heating means and the pressing means need to be larger in scale in proportion to the size of the plate and are therefore different to handle.